Photoionization of C₆₀: Effects of Correlation on Cross Sections and Angular Distributions of Valence Subshells

Abstract

Calculations of the photoionization cross section and asymmetry parameter, β, are performed at the density functional theory (DFT) and time-dependent density functional theory (TDDFT) levels for all 32 valence levels of C 60 . Accurate numerical results are obtained for the isolated molecule in icosahedral symmetry. A detailed analysis based on the comparison between the DFT and TDDFT results allows the identification of four types of resonances: the well-known confinement resonances of mainly geometrical origin, shape resonances native to the ionization channel, induced shape resonances, and autoionization resonances brought about by interchannel coupling, as well as their different prominence in cross section or asymmetry parameter. Generally, cross sections are enhanced at the TDDFT level, which includes contribution from the bound-state excitations from closed channels, neglected at the DFT level, and the effect persists even well above the highest ionization threshold. This effect is best seen in the total cross section, although not as dramatic as found from simpler models, probably due to the stiffer electronic structure inherent in the full molecular description. The effects of interchannel coupling on individual native resonances are rather less predictable, leading to both enhancement and decreases and often altering the details of the structure significantly. A comparison with the previous accurate total cross-sectional calculations, as well as with the available experimental data, is very good for cross sections but slightly inferior for β's. The results reported can serve as a reference to compare the effects of different environments on C 60 , as well as chemical substitution, notably endohedral fullerenes.

VUV photoionization dynamics of the C60 buckminsterfullerene: 2D-matrix photoelectron spectroscopy in an astrophysical context

“…Both bands suggest the presence of shape resonances above the ionization threshold, around 1-3 eV KE. 65,82 The partial cross sections of the first two bands have been previously calculated by Colavita et al 30 and more recently by Ponzi et al 65 The shapes differ quite significantly (see Fig. 9) and they, furthermore, do not account for all the shape resonances that are evident in our measurement.…”

“…theoretically obtained values. 65,82 The recent results from Ponzi et al 65 predict a much more negative value of b for the HOMO 6h u band for slow photoelectrons (KE o 4 eV), while the agreement for faster photoelectrons is more reasonable. As for the second band, the calculations predict negative anisotropy values ranging from À0.7 to À0.1 for both 6g g and 10h g states in our photon energy range, while our experimental values start at À0.5 at 10.5 eV and progress towards slightly positive values at 13.5 eV.…”

“…Within the second band there are two electronic states anticipated which are labeled 6g g and 10h g . 30,64,65 Depending on the theory, it is not clear whether the 6g g state or the 10h g state is higher in energy. While Zakrzewski et al predict the 10h g state to be 30 meV lower in energy than 6g g , Colavita et al predict the 10h g state to be 110 meV higher than the 6g g state.…”

“…We can only speculate on the origins of this broad resonance. It is possible that shape or confinement resonances, which have been calculated to be present in the continuum, 65 are stronger in the cation due to the increased attractiveness of the potential as felt by the departing electron. Further manipulation of the 2D photoelectron matrix can also reveal the partial photoionization cross sections of the first two bands up to 3.3 eV KE (i.e.…”

Continuum Electronic States: The Tiresia Code

“…Photoionization of C has been investigated several times [ 34 , 35 , 36 , 37 ]. Illustrative results from recent studies are reported in Figure 2 .…”